A common technique for extending voice services over a digital communication link involves terminating opposite ends of the link with a foreign exchange office (FXO) and foreign exchange subscriber (FXS) circuit, so that station lines from a private branch exchange (PBX) or key telephone system may be extended to a distant location. One of the features offered by many PBX systems, such as those commonly used in places of business and at hotel/motel locations, is the ability to controllably illuminate a telephone message-waiting light to alert the customer to a voice mail message.
When telephone station lines are to be extended over a digital communication link, such as a T-1 data rate link, using an FXO/FXS channel unit combination, the customary mechanism for energizing a message-waiting light has involved the use of frequency shift keyed (FSK) signaling, such as described, for example, in Bellcore document TA-NWT-001401, issue 1, and US WEST document 77335, issue A. Unfortunately, this conventional approach requires the installation of a modem at each end of the link for encoding and decoding the FSK signal, entailing an unacceptable cost.